1. Field of the Invention
The present invention relates to a method and apparatus for preventing damage to boiler ignitors caused by exposure to extreme heat, the adhesion of flyash and clinkers, and electrical arcing. The method and apparatus involves passing cool air over the ignitors while the boiler is ignited and operating.
2. The Background of the Invention
One of the most complex and time consuming processes in the operation of coal-fired electric generating plants is the start-up of those plants. One of the major difficulties in the start-up involves igniting coal which is burned by the plant. In order to assure proper ignition of the coal it is necessary to go through a multiple step process which employs relatively complex and expensive equipment.
To initiate the burning of coal it is generally necessary to first ignite a more easily ignitable primer fuel. Thus, it is now common practice to inject into the plant's furnace atomized oil or diesel fuel, or, in the alternative, a stream of natural gas. The primer fuel is then ignited by introducing an electrical spark into the stream of primer fuel. Once the primer fuel stream is fully ignited, a stream of pulverized coal can be gradually introduced into the furnace along with the primer fuel while still maintaining combustion in the furnace. Eventually, combustion is totally sustained by the stream of coal, at which time the stream of primer fuel is discontinued.
One widely used method of igniting a coal stream in a coal-fired generating plant employs a specially designed ignitor device. One such ignitor is the Type MPO Lighter manufactured by Babcock & Wilcox Power Generation Group. This lighter comprises a means for injecting primer fuel, such as fuel oil, into a furnace, together with an apparatus for atomizing the oil. The lighter also includes an electrically operated ignitor probe which can be pneumatically extended into or retracted from the furnace itself. The lighter is connected to an electrical power source so that a spark can be produced at the end of the ignitor.
As mentioned above, the process of starting the coal fire within a generating plant is relatively complex. Generally, the oil injector gun is inserted into the boiler with the oil injector being powered by a pneumatic cylinder which is part of the lighter assembly. A stream of steam is then passed through the oil injector gun, which is an integral part of the ignitor, and out into the furnace. This is done in order to purge any unwanted residue from the system. When this is accomplished the ignitor probes are inserted pneumatically into the furnace. Once extended the ignitor probes are fired electrically for approximately 25 seconds in order to burn any residual fuel within the oil injector gun and piping. At this point the start-up process itself can commence.
In order to begin the start-up process, oil is injected into the furnace and is atomized by steam or air from a separate source. The ignitor probe fires until the atomized oil begins to burn. It will be appreciated that alternative primer fuels, such as natural gas, could be substituted in order to initiate combustion.
Once the primer fuel is sufficiently ignited, pulverized coal is blown into the furnace in the vicinity of the existing flame. The coal combines with the primer fuel and is burned. Gradually, the amount of coal blown into the furnace is increased and the primer fuel discontinued and a completely coal-fired flame is produced. When this occurs the injector probes and the oil injector gun are withdrawn from the furnace.
One major problem with this process, and one of the problems which the current invention solves, is that of the deterioration and eventual destruction of some of the operative elements of the ignitor probes. Indeed, it is generally found that ignitor probes are only usable for one plant start-up process. After a cycle involving start-up and operation of the plant it is found that the center electrode of the ignitor probe is no longer usable. Even when the probe is retracted, the intense heat under which the entire process takes place damages the electrodes. Typically the ignitors operate at about 1500.degree. F., and even when retracted are exposed to the intense radiant heat of the furnace. In addition, it is found that the end of the ignitor probe which contains the exposed electrodes often becomes coated with flyash and clinkers. As a result of the extremely hostile environment in which the ignitor probes must operate, it is not surprising that they are only used once and then replaced.
The cost of constantly replacing ignitor probes, however, is extremely high. In typical modern power plants, it is not unusual to have four banks of 10 ignitors, or a total of 40 ignitors. Each ignitor probe costs approximately $150, resulting in a cost for the probes alone of $6,000 per start up procedure. Added to the cost of the probes are the additional costs of taking the plant out of service, the resulting loss of generation of electricity, and the cost of labor required to change the probes. Primary among these costs is the loss of electric generation. It is estimated that the cost of taking a plant out of service is approximately $8,000 per hour. The probe replacement process can easily take 7 to 8 hours. As a result, it will be appreciated that each time the ignitor probes must be replaced, which is essentially each time a plant goes through the start-up procedure, the cost is in the range of $50,000 to $70,000.
It will be appreciated, therefore, that what is needed in the art are methods and apparatus for protecting the ignitor probes from damage in order to enable them to be reused. Specifically, it has been found that if the probes can be kept at a temperature in the general range of 200.degree. F., much of the expected damage can be prevented. If, in addition, flyash and clinkers can be kept off the probes, most probes can be maintained in relatively good condition. Therefore, it would be an advancement in the art if methods and apparatus could be provided which would keep the ignitor probes cool and at the same time keep flyash and clinkers away from the probes. It would be an additional advantage if existing ignitor assemblies could be retrofit with such an invention. Such methods and apparatus are disclosed and claimed in this application.